It was found that in the case of lap welding of thermoplastic material or sealing webs intended for lining landfills, the weld seams prepared without welding filler have the required optimal weld strength only when the weld thickness D.sub.N is between an upper limit D.sub.NO =D-0.2 mm and a lower limit D.sub.NU =D-0.6 mm, in which D is the sum of the thicknesses d.sub.1 and d.sub.2 of the unwelded material webs. The weld thickness D.sub.N is smaller than the sum D due to the fact that part of the plasticized material is pressed out of the welding gap by the pressing force applied by the pressure rollers on the partial areas of the material webs, which are heated to the welding temperature and thereby plasticized. If the required reduction in thickness or the upper limit D.sub.NO was not reached during welding, the material was not heated sufficiently in the welding gap, and consequently it was not sufficiently plasticized, as a consequence of which the weld seam did not have the necessary strength. In contrast, if the weld thickness was below the lower limit D.sub.NU, the material was heated too intensely, as a consequence of which the strength was impaired by notch effect at the weld margins. The above-mentioned limit values for the weld thickness are independent from the thickness of the material webs usually used to line landfills.
In the process for carrying out welding processes on thermoplastic material webs intended for lining landfills, which has been known from DE 40 20 704 C1, the welding process is carried out automatically at an essentially constant welding gap temperature by measuring the heated wedge temperature regulation by influencing the energy supply to the heated wedge in the case of deviations of the actual temperature value from the set value, and additionally regulating the feed when the actual temperature value exceeds or drops below a set point range.
Since the heated wedge temperature, which represents the controlled variable in this regulation, does not make it possible to obtain direct information on the quality of the weld seam, the required weld seam quality can be achieved only by adjusting the actual value of the welding temperature and the settings for the feed speed and the pressing force of the pressure rollers as accurately as possible to the influences of the environmental conditions, and especially to the starting temperature of the material webs, as well as to the material-specific properties. In a simple version of the regulating system, this is done by the operator. In an automated version, the environment-specific influential factors are determined by corresponding sensors, after which the computer of the regulating system determines the correct welding temperature set value by means of the measured values and the influencing variables entered via a keyboard.
Another requirement for achieving the required weld seam quality is that the actual value of the welding temperature be determined, if possible, without delay. However, this requirement is satisfied only very imperfectly in the simple version, because the welding temperature is measured within the heated wedge rather than in the welding gap so that the welding gap temperature, which is actually the temperature that is of interest, is determined only indirectly and with a time delay. In contrast, the dead time caused by the measuring technique in the control circuit is compensated in the automatic version by the measure that the regulating system is able to respond prospectively to a greater change in the starting temperature of the material webs due to the fact that the starting temperature of the material webs is measured at a spaced location in front of the welding site.
While the determination of the set value is circumstantial in the simple version, and the regulating system responds very sluggishly because of the time delay in temperature measurement, the automated version requires a relatively high technical effort, and it is also subject to the restriction that the determination of the starting temperature of the material webs is susceptible to disturbances due to contamination of the material webs or to moisture.